# Dopplerâ€™s Effect of Light

The phenomenon of apparent change in frequency (or wavelength) of the light due to relative motion between the source of light and the observer is called Dopplerâ€™s effect.

If v = actual frequency, vâ€™= apparent frequency, v = speed of source wrt stationary observer, c = speed of light.

# Source of light moves towards the stationary observer

When a light source is moving towards an observer with a relative velocity v, then the apparent frequency (vâ€™) is greater than the actual frequency (v) of light. Thus apparent wavelength (Î»â€™) is lesser the actual wavelength (Î»).

and

For v << c
• Apparent frequency,
• Apparent wavelength,
• Dopplerâ€™s shift: Apparent wavelength < actual wavelength,

So spectrum of the radiation from the source of light shifts towards the violet end of spectrum. This is called violet shift

Dopplerâ€™s shift,
• The fraction decrease in wavelength

# Source of light moves away from the stationary observer

In this case, vâ€™ < v and Î»â€™ > Î»

and

For v << c
• Apparent frequency,
• Apparent wavelength,
• Dopplerâ€™s shift: Apparent wavelength > actual wavelength,

So spectrum of the radiation from the source of light shifts towards the red end of spectrum. This is called red shift

Dopplerâ€™s shift,
• The fractional increase in wavelength